Dress comfortably. If you are uncomfortable, people can tell, and it makes you less attractive. You might think those skinny jeans look smashing on you, but if they cut off the circulation in your legs because they're actually girls' jeans, it's probably not something you should be wearing.

• That being said, don't go too tight or too loose. It is important to make your body looked proportioned. Avoid wearing tight jeans and a loose shirt. This causes you to look top-heavy. Wear slim-fitting (not tight!) jeans and shirts that seem tailored.
  
• A good shirt will fit with the shoulder seam on the curve of the shoulder, and the cuff stopping just before the curve of the hand (where the hand meets the wrist).
  
• If you choose to wear shorts, choose a slightly wider leg than you would on a pair of pants. Shorts should stop somewhere between the top and middle of the knee. This provides a balance, as you only have half as much length

Make sure whatever you wear fits properly and looks good on you. The fit is the most important aspect of any article of clothing. No matter how cool the style is, if it doesn't fit you, do not wear it.

• Remember that cotton clothing will shrink a small amount the first time you wash and dry the fabric. If you plan on drying your cotton clothes, look for sizes that are slightly too big to accommodate shrinkage. If you dry clean certain items, you won't have to worry about it.
  
• For shirts, the seam at the beginning of the sleeve should start right where your shoulder ends. The length of the shirt should extend beyond the belt but not beyond the buttocks.
  
• For pants, the waist should fit you comfortably right above the hips. The pant leg should extend down at least to the top of your shoe, but not to the ground.
  
• Don't be afraid to get some of your nicer clothes tailored. If you buy your clothes at a reputable place, they'll have a tailor on hand. The tailor will get your measurements and talk with you about what kind of look you want to achieve. For a small fee (or often none), your clothes will come back looking like they fit only you.

One of the best investments a guy can make is a simple white button-up shirt that fits like it is tailored. Every guy looks great in it, and it can be dressed-up with slacks and a tie or dressed-down with jeans and sneakers. The same goes for a good blue oxford-cloth shirt.

• European cut shirts are a bit different from American cut. The European cut is a little slimmer and more form-fitting along the sides, while American cut is a little baggier, allowing for more room.

Reposted from http://www.wikihow.com/Dress-Well-As-a-Guy